1. Field of the Invention
The present invention relates to a spark plug for an internal combustion engine and a noble metal chip for the electrodes of the spark plug.
2. Description of the Related Art
With regard to spark plugs for internal combustion engines, it is a continuing need to improve the lifetime (i.e., to improve the consumption resistance) and the performance (i.e., to improve the spark flyability and the ignitability). To this end, attempts have been made to use a noble metal chip, particularly of Pt, as a discharging element of the spark plug for an internal combustion engine.
Recently, there is an increasing need to further improve the lifetime and the performance. To this end, studies have been conducted to use a noble metal chip made of Ir having a melting point higher than that of Pt.
Although Ir advantageously has a melting point of 2454.degree. C., which is much higher than that of Pt, it also has a disadvantage that the volatilization consumption is undesirably accelerated at elevated temperatures above 900.degree. C.
To suppress the volatilization consumption of Ir, Japanese Unexamined Patent Publication (Kokai) No. 2-186579 proposed introduction of Al, Y or other elements having a strong affinity for oxygen and precipitation thereof to form an oxidation-protection film over the surface of an Ir electrode, thereby preventing the volatilization consumption of Ir.
However, the proposal had a drawback that the oxidation-protection film is composed of alumina, yttria, or other ceramics, which provides poor bond with the Ir noble metal and causes spalling thereof.
Japanese Unexamined Patent Publication (Kokai) No. 2-49388 proposed addition of Pt to Ir, but could not provide a satisfactory noble metal chip having a sufficiently improved consumption resistance and heat resistance at high temperatures.